A radio system using microwaves (in particular, quasi-millimeter wave band), which tends to be influenced by rainfall attenuation, adopts adaptive type coding/modulation (henceforth, referred to as adaptive modulation) for the purpose of improvement of availability of radio links. Adaptive modulation is a technique which enables wide band and stable radio communication using radio waves as the communication medium influenced by external causes. And, adaptive modulation is technology which varies a modulation method according to a change in the propagation environment of radio waves, for example, fluctuation of bit error rate. Adaptive modulation can suppress increase of the error rate by performing low bit rate modulation at time of deteriorated condition of the propagation environment and avoid deterioration of the communication quality from being occurred. Adaptive modulation can make the transmission rate high by performing high bit rate modulation at time of good condition of the propagation environment.
As the modulation methods, there are BPSK (Binary Phase Shift Keying), QPSK (Quadrature Phase Shift Keying) and 16 QAM (16 Quadrature Amplitude Modulation) or the like. The number of bits that can be transmitted per one symbol in each modulation method is 1 bit in case of BPSK, 2 bits in case of QPSK and 4 bits in case of 16 QAM. And, the bit error rate characteristics in each modulation method deteriorate in the order of BPSK, QPSK and 16 QAM.
Accordingly, when the propagation environment of radio waves is bad, BPSK and QPSK which are the modulation methods having strong error tolerance are used. Further, when the propagation environment of radio waves is good, it gives priority to the transmission rate over the error tolerance, and for example, 16 QAM which indicates the double transmission rate of QPSK is used for the modulation method.
On the other hand, there is MPLS as technology for composing a network.
MPLS is technology which has introduced a concept of a label in order to realize flexible path control which is free from a framework of layer 3 in the network which is composed of routers. In other words, MPLS is technology which attaches a label to a place located in the middle of a data link header of layer 2 and an IP (Internet Protocol) packet header of layer 3, and transmits packets at high speed based on information included in the label. Therefore, MPLS can realize routing of packets which is not constrained by an IP address.
The traffic between edge nodes of MPLS is transmitted using a connection, called a label path (henceforth, referred to as LSP: Label Switched Path), which is a fixed path. Therefore, it can be said that MPLS has introduced connection-oriented mechanism into an IP network which is connectionless. And, MPLS can explicitly establish and control a path having an idle bandwidth more than a requested bandwidth using a signaling protocol in each link through which this LSP passes. As the signaling protocol, there is CR-LDP (Constraint-based Routed Label Distribution Protocol) and RSVP-TE (Resource Reservation Protocol Traffic Engineering) or the like. In other words, in MPLS, the technology which effectively uses bandwidth resources of the whole network and guarantees communication quality is specified.
MPLS calculates a path which is capable of providing a required bandwidth for communication using a path calculation algorithm which takes constraints of an available bandwidth or the like into account. The path obtained in this way is reserved in each of routers prior to performing communication. That is, the bandwidth corresponding to the path is secured in each of routers. As a result, the quality of communication is guaranteed and the bandwidth resources of the network are used more effectively.
The bandwidth control technology which controls a communication bandwidth while suppressing influence given to the communication in a communication system which is constructed by connecting radio devices corresponding to adaptive modulation and MPLS routers is disclosed in patent document 1 (Japanese Patent Application Publication No. 2006-287549). This technology solves such a problem that the data, which should be able to be transmitted with a reserved bandwidth, cannot be transmitted actually due to decrease of a communication bandwidth by adaptive modulation if adaptive modulation by the radio device is performed regardless of the communication bandwidth being reserved on the MPLS router.
The MPLS router disclosed in patent document 1 searches for an alternative path to which a communication path corresponding to the decreased amount of the communication bandwidth is to be diverted, when notification of a change in the modulation method to a low order is received from the radio device. And, when having found the alternative path, the MPLS router changes the modulation method in the original radio circuit while using the alternative path. Even if not found an alternative path, the modulation method is changed after performing disconnection of the communication path. In other words, when performing the change in the modulation method to a low order, when having found the alternative path, communication of the communication path which cannot be accommodated anymore due to the change in the modulation method to a low order is maintained via other adjacent MPLS routers. Also, even if not found an alternative path, it controls the communication bandwidth while suppressing influence given to the communication by performing release processing first to the communication path which cannot be accommodated any more due to the change in the modulation method to a low order.
Patent document 2 (domestic re-publication of PCT international publication for patent applications WO 2006001308) discloses a radio link system which can reconfigure a path depending on fluctuation of the bandwidth which occurs in association with a change in the propagation environment of the radio link. In this system, an arbitrary radio node device among a plurality of radio node devices detects the transmission state of its own radio link that is connected to itself, and changes the modulation method of the radio link based on the detected transmission state of the radio link. And, this radio node device transmits information indicating the changed modulation method and the number of paths using the radio link in which the modulation method has been changed to a path control device. The path control device detects a path which requires a path change based on the information indicating the modulation method and the number of paths using the radio link in which the modulation method has been changed transmitted from the radio node device, and determines a new path from the state of the whole radio links.
Further, an MPLS network has a restoration function which calculates an alternative path for fault relief to a fault occurring in the MPLS network and performs processing for establishing a new LSP. In the MPLS network, when a fault has occurred in a link which an LSP uses, information about the fault event is notified to an initiator node of the LSP. A Notify message of RSVP (Resource Reservation Protocol) is used for this notification, and by this, the fault event is notified directly from a node of the fault section to the initiator node which performs the fault relief. The initiator node which has received the Notify message calculates the alternative path and newly establishes an LSP. The fault relief processing method of MPLS network like this is disclosed in patent document 3 (Japanese Patent Application Publication No. 2005-333383).
Technology which patent document 3 discloses eliminates a possibility that a different path included in the fault section is selected as the alternative path, and it enables to perform the fault relief processing stably. According to this technology, when a fault has occurred at any path of the MPLS network, the Notify message which is fault event information is transmitted from a node which has detected the fault. When the Notify message is received, the node which performs the fault relief begins to measure protection time by using this reception as a trigger. During this protection time, LSA (Link State Advertisement) of a routing protocol of OSPF (Open Shortest Path First) is collected. When the protection time is completed, the node which performs the fault relief executes alternative path calculation based on the Notify message and LSA of OSPF, and performs the fault relief by performing the restoration processing.